Cache feature

This branch implements a cache for the sensor data. The idea is to make this somewhat high performance and also lower the cpu resources needed for the application by creating a file under /tmp (configurable) which stores a JSON cache based on the PID of the running process - this way you can have multiple threads and they wont lock or create contention.

Caching can be disabled, by setting CACHE_EXPIRE_TIME to 0, this will instruct the code to refresh its data from the sensor every time.